Purpose China has developed new fire-resistant steel grades, particularly Q420FR and Q460FR. Evaluating the residual mechanical properties of these steels after fire exposure is essential for post-fire damage assessment and potential structural reuse. This study aims to investigate the post-fire mechanical behavior of Q420FR and Q460FR steels over a wide temperature range. Design/methodology/approach Experimental tensile tests were conducted on Q420FR and Q460FR steel specimens exposed to temperatures ranging from ambient conditions to 800°C, followed by natural air cooling. The residual elastic modulus, yield strength, ultimate tensile strength and ultimate strain were systematically evaluated. The test results were compared with those of other structural steels reported in the literature. Based on the experimental data, constitutive equations were proposed to predict the post-fire mechanical properties of the two steels. A reliability analysis was performed to assess the validity of the proposed models. Findings The results indicate that both Q420FR and Q460FR steels exhibit limited degradation in mechanical properties at temperatures up to 600°C. Beyond this threshold, exposure temperature has a pronounced influence on residual strength and ductility. Compared with conventional structural steels, Q420FR and Q460FR demonstrate superior strength retention and deformation capacity after high-temperature exposure. The proposed constitutive equations show good agreement with experimental results and provide conservative predictions of post-fire strength and stiffness. Originality/value This study provides a comprehensive experimental evaluation of the post-fire mechanical properties of Q420FR and Q460FR steels and establishes validated constitutive models for predicting their residual performance. The findings offer valuable guidance for post-fire assessment, design and reuse of fire-resistant steel structures.
Yang et al. (Tue,) studied this question.